This invention relates to an illuminating light source for a color image recording device.
Recently, a device has been proposed in the art which reads a color original photo-electrically, and prints out a color image through various color image processing operations.
In a color image reading device of this type, a daylight light source such as a fluorescent lamp or halogen lamp which outputs a large quantity of light on low electric power is used to irradiate the surface of an original, and light reflected therefrom is detected with a contact type CCD (charge-coupled device) line sensor for instance.
FIG. 13 is a sectional view of an ordinary fluorescent light used for reading an original. In the fluorescent lamp, a fluorescent film is formed on the inner wall of a glass tube 2, which is filled with mercury vapor. When, under the condition that the mercury vapor pressure in the glass tube is increased with the mercury vapor heated by a heater (not shown), electric discharges are induced therein, ultraviolet rays high in intensity (257 nm) are emitted from the mercury vapor in the glass tube, thus exciting the fluorescent substance of the fluorescent film 4 to emit fluorescence. In addition, the mercury vapor produces a bright line spectrum in a visible range. In this case, the bright line from the mercury vapor in an ultraviolet range is scarcely emitted outside, being absorbed by the glass tube 2. However, in the fluorescent lamp shown in FIG. 13, the fluorescence is radiated in all directions, and therefore the quantity of light applied to the surface of the original is not so large.
In order to overcome this difficulty, a fluorescent lamp as shown in FIG. 14 has been proposed in the art. That is, as shown in FIG. 14, in order to increase the quantity of light applied to the surface of an original, a reflecting film 3 is formed on the inner wall of a glass tube except its part (aperture), and a fluorescent film 4 is formed on the reflecting film 3. In the fluorescent lamp thus constructed, the reflecting film is located behind the fluorescent film. Hence, the fluorescence reflected is allowed to go towards the original through the aperture which is not covered by the reflecting film 3. Thus, the fluorescent lamp is high in light emitting efficiency, providing a large quantity of light.
FIG. 15 is a diagram showing a spectral characteristic of a so-called "three-wavelength type fluorescent lamp". The specific feature of the fluorescent lamp resides in that the quantity of light is large. However, since the bright line is great, a so-called "metamerism phenomenon" takes place in which the color read is different from that which is observed with the human eye.
Color originals to be read are prints, pictures, etc. FIG. 16 is a diagram showing spectral characteristics of a print and a picture in the case of skin color, and FIG. 17 is a diagram showing spectral characteristics of a print and a picture in the case of sky-blue color. In these figures, reference character I (.lambda.) designates the spectral characteristics of the prints, and P (.lambda.) the spectral characteristics of the pictures.
When such originals are observed with the human eye under a standard light source which is of the order of 6700K in color temperature and flat in spectral characteristic, they are seen as if they were the same in color even if they are different in spectral characteristic, because the human eye perceives colors with so-called "three stimulus values (three original stimuli required for color matching with the color stimulus of a specimen). Hence, when, in a color image recording device, the prints and the pictures shown in FIGS. 16 and 17 are read with a CCD line sensor, the same R, G and B values must be outputted.
In the case where the aperture type fluorescent lamp as shown in FIG. 14 is used, the bright lines in the visible range are emitted out of the tube directly, being not absorbed. Therefore, in the case where the bright lines are of long wavelengths as in the case of prints or pictures which are large in spectral characteristic difference, then the difference is read with emphasis, as a result of which the same R, G and B values cannot be outputted; that is, a different color image is reproduced. This is more serious with the three-wavelength type fluorescent lamp as shown in FIG. 15 because the bright line is great.
In order to eliminate this difficulty, it is necessary to reduce a ratio of bright line to fluorescence, which is a ratio of the output power of the bright line spectrum produced by the mercury vapor to that of the fluorescence emitted by the fluorescent material. As shown in FIG. 3, a fluorescent lamp generally has a heat sink 222d at the middle of its tube because of the following reason: A light emitting efficiency is a function of mercury vapor pressure.
Not only when the mercury vapor pressure in the tube is excessively high but also when it is excessively low, the light emitting efficiency is decreased, and an aimed quantity of light cannot be obtained. Therefore, a heat sink is provided in the tube at the middle to radiate heat thereby to decrease the saturated vapor pressure. As a result, the unwanted vapor at both ends of the tube is moved towards the center, whereby the vapor pressure in the tube is made suitable.
When an aperture type fluorescent lamp (15.5 mm in tube diameter) as shown in FIG. 14 is turned on and off at a cycle of 10 seconds on and 10 seconds off with a lamp current 400 mA, the distribution of luminescence in the longitudinal direction of the tube is as shown in FIG. 18.
When, as shown in FIG. 19A, the number of copies is increased with the middle portion of the tube held at a temperature of 45.degree. C. by the heat sink, then the end portions of the tube are increased in temperature by discharge heating, and the mercury vapor pressure is increased. As a result, at each of the end portions the quantity of light is decreased as shown in FIG. 19B.
A lamp as shown in FIGS. 20 and 21 is known in the art which are a top view and a side view thereof. The lamp is formed as follows: Bases 105 are connected to both ends of a fluorescent lamp 101 which comprises a glass tube embraced by a lamp heater 102 and an aperture 104 for radiation of light. The bases 105 have cuts, in which conductors 106 are provided. In connecting the lamp to its sockets 109, as shown in FIG. 22A the two ends of the lamp are inserted into the sockets 109 from above with the conductors 106 of the bases 105 held oblique, and then the lamp is turned 45.degree. about its axis so that it is fixedly connected to the sockets 109 as shown in FIG. 22B.
Furthermore, a lamp heater 102 is combined with the lamp to stabilize the quantity of light of the latter as shown in FIG. 23. Under this condition, the bases of the lamp are connected to the sockets by turning it. In this operation, the heater may be displaced to cover the aperture 104. This difficulty occurs with high probability. And it is rather difficult to handle the lamp so that the latter is correctly connected to the sockets. Furthermore, a skilled person must carry out the operation of coupling the heater to the lamp while confirming the position of the aperture. Therefore, an unskilled person may install the lamp incorrectly to start the image recording device, thus causing a printing error.